This invention relates to a control device for a single phase induction motor, more particularly an improved control device for controlling the motor for use in driving an apparatus, such as a sewing machine, which is operated intermittently and the load is intermittently applied to the motor.
Conventionally, a sewing machine is driven by a single phase induction motor having a magnetic field circuit containing a main winding and an auxiliary winding; wherein a capacitor is connected to the auxiliary winding to advance the phase of the current passing through the auxiliary winding to facilitate the starting of the motor. The field circuit is normally so designed such that a relatively large amount of the current is allowed to pass through the main winding and thus the major portion of the torque produced by the motor is generated by the main winding. In an earlier type of the single phase induction motor, a switch is provided in the auxiliary winding such that the switch is caused to open as soon as the motor is started and its speed has reached a certain level to cut off the current supplied to the auxiliary winding, leaving the motor to operate solely on the main winding. When either one of such conventional single phase induction motors, with or without a cut off switch, is used for driving a sewing machine, a magnetic clutch is normally employed to couple the motor with the sewing machine. This is because the sewing machine operates intermittently and the motor must be disconnected from the sewing machine instantly when the sewing machine stops, and on the other hand, the motor must be capable of being coupled to the sewing machine immediately when one wishes to operate the sewing machine.
A problem arises as the operation of the motor is maintained solely by the main winding, which draws a relatively large amount of the electric current even when the magnetic clutch is disengaged and the load is not applied onto the motor; a considerable amount of the electric energy is still consumed by the motor while the sewing machine is not operating and the motor is left idling, and such an excessive consumption of electric energy tends to generate heat and noise.
In view of the above-mentioned problems with the conventional single phase induction motor, this application offers an improved control device which reduces electric energy consumption while the motor is operating under a no-load condition.